Lazy binding of variables for global 'filename' default

Without this commit, the default value of the config 'filename'
attirbute was dependant on the values of the @version and @suffix
variables pre-merge time. Since these variables need only be set at
either global or local levels, there is no way to resolve their usage in
this default until the merge.

This code changes the default value to use the @version and @suffix
variables of the local object (which will be post merge in this case) in
the event the @filename variable is set to
`Vagrant::Plugin::V2::Config::UNSET_VALUE`.
Refactor 8a7efe5

Commit 8a7efe5 added a late binding method for the 'filename'
configuration default. This commit cleans up and refactors 8a7efe5 to
change from an early return to full conditional. It also removes the
commented code that was replaced by this new late binding method.
Lastly, it cleans up a spelling mistake in the comments.
Merge pull request #19 from trlinkin/filename_binding

Lazy binding of variables for global 'filename' default